Solid state fermentation has been practiced for centuries, most often in connection with food production, and can be defined as a technique for growing microorganisms, such as fungi, yeast and bacteria, on moist solid substrates. In recent years, there has been a resurgence of interest in solid state fermentation and its applicability to the production of enzymes, metabolites and organic compounds. Solid state fermentation devices provide several advantages over the commonly used process of submerged fermentation in product yield, cost and ease of use. Despite their economic advantages, the commercialization of solid state fermentation devices for industrial use has been limited for lack of efficient and practical designs.
A wide variety of solid state fermentation devices have previously been described (for review see, Larroche et al., "Special Transformation Processes Using Fungal Spores and Immobilized Cells", Adv. Biochem. Eng. Biotech., (1997), Vol 55, pp. 179; Roussos et al., "Zymotis: A large Scale Solid State Fermenter", Applied Biochemistry and Biotechnology, (1993), Vol. 42, pp. 37-52; Smits et al., "Solid-State Fermentation-A Mini Review, 1998), Agro-Food-Industry Hi-Tech, March/April, pp. 29-36). These devices fall within two categories, those categories being static systems and agitated systems. In static systems, the solid media is stationary throughout the fermentation process. Examples of static systems used for solid state fermentation include flasks, petri dishes, trays, fixed bed columns, and ovens. Agitated systems provide a means for mixing the solid media during the fermentation process. One example of an agitated system is a rotating drum (Larroche et al., supra).
A major problem in both static and agitated solid state fermentation systems is obtaining efficient removal of heat that is generated during the fermentation process. One method of heat removal employed by numerous solid state fermentation systems is aeration. The disadvantage of using aeration as the means of heat removal is that not only is heat removed, but water is also evaporated from the solid matrix, leading to desiccation of the substrate. Constant aeration also makes it more difficult to maintain a stable environment inside the bioreactor with respect to oxygen and carbon dioxide concentrations. Another means of avoiding heat build up is mixing the substrate bed. Unfortunately, mixing during fermentation leads to damage of the cells and gross aggregation of substrate particles. Aggregation of substrate leads to inhomogeneities in local substrate temperature resulting in local differences in biomass growth and activity. These problems are compounded in the large scale systems often required for industrial preparation of certain products. The large scale practice of solid state fermentation using the devices available in the art has the additional disadvantage of being labor intensive.
The steps involved in solid state fermentation include, 1) sterilization of the cultivation device and the cultivation media, 2) inoculation of the cultivation media with the microorganisms, 3) cultivation of the microorganisms 4) extraction of biological products from the cultivated microorganisms, and 5) post extraction treatment of the waste materials and the cultivation device. It is also desirable that the cultivation system provide a mechanism whereby the growth environment during the cultivation process is precisely controlled such that specified conditions are maintained throughout the cultivation process. None of the devices available for solid state fermentation to date provide for carrying out all of the steps required for solid state fermentation in a single fermentation device. Up to now, the practice of solid state fermentation has involved carrying out multiple manipulations which are both tedious and impractical. Such manipulations often risk exposing the cultivation environment to contaminants from outside the cultivation environment, preclude the ability to efficiently and precisely control the cultivation, and lead to reduced product quality and/or yield.
There exists a need for a compact reactor that combines all the operations involved in solid state fermentation into a single device capable of operating in a contained manner and controlling the environment within the bioreactor without inhibiting growth of the microorganism. Furthermore, there exists the need for a device that will allow homogeneous addition of chemicals and nutrients to a bioreactor without contamination.